The potential profile across the channel of an operating sexithiophene-based field-effect transistor (FET) was investigated using an atomic force microscope with a conducting probe. A high impedance electrometer recorded the probe potential when it was placed in contact at fixed points with the channel surface. Tapping mode images taken with the same probe before and after individual point contact measurements verified that no damage was done to the device and allowed correlation of the potential profile with the device architecture. For any given source-drain bias, most of the potential drop occurred at the source and drain contacts, meaning the FET was contact limited. Moreover, the potential drop was not fixed but depended on the applied drain and gate voltages. This study demonstrates the utility of potential profiling for identifying high resistance bottlenecks to charge transport in organic-based devices.